126 UNITED STATES NATIONAL MUSEUM BULLETIN 29 6 



The shape of the burrow of barnacles of the genus Trypetesa is 

 highly dependent upon thicknesses of the host shell, a lateral orienta- 

 tion being more pronounced in thinner shells. 



T. lateralis, with a completely lateral orientation, is adapted to 

 inhabit very thin Tegula shells. It is further modified by the presence 

 of a unique external mantle flap, which penetrates to the outside 

 surface of the shell, opposite the point of entry into the shell. The 

 purpose of this second perforation m the shell is to maintain water 

 currents, and is intimately associated with the presence of the males 

 on the females. The external mantle flap is considered to be an adapta- 

 tion to allow males to establish themselves underneath the laterally 

 flattened females (see page 15). 



The females of Trypetesa lampas are oriented perpendicularly to 

 the surface of the host shell, necessitating a much deeper burrow 

 than those of the females of T. lateralis, which are oriented parallel 

 to the surface of the host shell. Measurements of the thickness of 

 the host shells at the site of the burrows would be useful in determin- 

 ing the possible significance of this orientation. 



T. habei and T. nasmrioides are so variable in this characteristic 

 that no attempt will be made to compare these species at this time. 



Three shell fragments of Buccinum undatum from Plymouth, 

 England, containing 23 specimens of T. lampas, and 10 shells of Tegula 

 brunnea and Tegula funebralis from Moss Beach, California, with 10 

 specimens, were measured as close to the apertural slit as possible, 

 perpendicular to the surface of the shell, with outside direct-reading 

 micrometers. The average thicknesses, as given in table 2, are 

 2.19 mm for T. lampas and 0.98 mm for T. lateralis. 



It is interesting to compare the shape of the burrow and orientation 

 within the shell of T. lampas (fig. 34ab) and T. lateralis (fig. 26ab) 

 with their respective shell thicknesses. T. lampas utilizes its full shell 

 thickness much more completely than does T. lateralis. Burrows of 

 the adult T. lampas females invariably leave a thin shell layer between 

 the barnacles and the outside of the shell. T. lateralis females actually 

 penetrate the outside surface of the shell with a minute hole from 

 the external mantle flap, but the rest of the body lies well away from 

 this surface. 



It appears that the lateral orientation of T. lateralis is an adjustment 

 mechanism to adapt to the much thinner shells of its host. 



It is possible that the protuberant button or prickly pads on the 

 distal end of the second segment of the first two pairs of terminal cirri 



